Conventionally, an optical particulate separator is known which sorts particulates such as grains or resin pellets into non-defectives and defectives by blowing grains or resin pellets using air-jets or removes foreign matter and the like mixed in particulates using air-jets.
This type of particulate separator sorts particulates falling along a predetermined trajectory from an end of a transport path, by blowing off and removing defectives and the like by air-jets according to a detection signal of the defectives and the like.
The particulate separator described above blows off defectives and the like by air-jets from particulates falling continuously in large quantities, and in order to blow off only the defectives and the like accurately without involving other particulates, it is necessary to equip an air-jet nozzle with a responsive valve.
Thus, the present applicants have developed a piezoelectric valve which opens and closes a valve at high speed using a piezoelectric element (see Patent Literatures 1 to 3).
The piezoelectric valves described in Patent Literatures 1 to 3 include a displacement magnifying mechanism adapted to magnify small displacement of the piezoelectric element based on the principle of leverage.
The optical particulate separator equipped with the air-jet nozzle which uses the piezoelectric valve accurately blows off defectives and the like with reduced risk of involving and blowing off non-defectives and the like because the piezoelectric valve is superior to conventional solenoid valves in responsiveness during valve opening and closing.
Now, the optical particulate separator has a problem in that stable sorting action is not available because the piezoelectric valve moves a valve disc via the displacement magnifying mechanism, causing vibration of the valve disc, which results in fluctuations of air jet volume from the nozzle.
Thus, to solve the above problem, Patent Literature 2 describes a piezoelectric valve which applies two-step drive voltages to a piezoelectric element.
The piezoelectric valve described in Patent Literature 2 applies a first-step voltage to the piezoelectric element, timed to drive and open the valve disc, and applies a second-step voltage higher than the first-step voltage to the piezoelectric element, timed to prevent the vibration of the valve disc occurring when the valve opens. This makes it possible to limit fluctuations in an amount of ejected gas after the valve is opened and thereby stabilize gas supply at an early stage.
However, the piezoelectric valve described in Patent Literature 2 has a problem of inferior responsiveness during valve opening compared to when the drive voltage applied to the piezoelectric element is a rectangular single-step voltage.
To solve the problem with the piezoelectric valve described in Patent Literature 2, Patent Literature 3 describes a piezoelectric valve which includes a signal generator adapted to generate a signal made up of a prepulse and a main pulse and applies a single-step drive voltage of a specific voltage value to a piezoelectric element based on the signal generated by the signal generator.
The piezoelectric valve described in Patent Literature 3 applies a drive voltage of a specific voltage value to the piezoelectric element based on the prepulse, timed to open a valve disc, and applies the drive voltage to the piezoelectric element based on the main pulse, timed to prevent the vibration of the valve disc occurring when the valve opens. This makes the piezoelectric valve superior in responsiveness during valve opening to the piezoelectric valve described in Patent Literature 2.
However, the piezoelectric valve described in Patent Literature 3 takes a longer time to limit fluctuations in the amount of ejected gas after the valve is opened than the piezoelectric valve described in Patent Literature 2 and leaves room for further improvement in order to stabilize gas supply at an early stage.